The current epidemic of obesity has occurred despite the existence of a body weight regulatory system, which, for most of mankind's history, has matched energy intake with energy expenditure sufficiently to avoid obesity. This suggests that the primary cause of the current obesity epidemic is not genetic, but may be due to an environment in which the energy balance regulatory system cannot function with sufficient precision to keep the population lean. In this study, we propose research aimed at understanding how high fat/energy dense diets can promote obesity by affecting the precision of regulation of energy and fat balance. It is our intent to ultimately identify dietary and physical activity patterns that are associated with increased precision of energy balance regulation and which can prevent development of obesity. Laboratory data suggest that high fat diets promote obesity by increasing the probability of over-consumption of total energy. We plan to systematically examine the relationship between dietary fat and energy intake across a range of diet compositions in sedentary subjects. While this has been done for diets with extreme variation in dietary fat (i.e. <20% vs. >40-60%) it has not been done for dietary fat content within the range of usual consumption of U.S. adults (i.e. 20-40% fat diets). We hypothesize that this relationship will not be linear and that there will be a threshold level or a range of dietary fat associated with a low probability of increased energy intake and positive energy balance. This information will be useful in developing dietary guidelines for obesity prevention.